Along with developments and applications of technologies, liquid concentration detecting devices are needed in various fields, such as etchant formulation in a wafer manufacturing process or vehicle emission in an environment protection field.
Taking the vehicle emission for example, in order to comply with an environmental emission standard, heavy-duty diesel vehicles internationally tend to adopt a selective catalytic reduction (SCR) technique to reduce emissions of nitrogen oxide (NOx) in the exhaust. In detail, an SCR system includes vehicle urea and a SCR device. The vehicle urea is injected into the combusted waste gas from a urea tank and decomposes into ammonia (NH3), such that a chemical reduction occurs to NOx by NH3 in the SCR device to transform the NOx into Nitrogen and water which cause no influence to the natural environment. Accordingly, the SCR system needs to consume a reactant (i.e., the vehicle urea) for normal operation. A car urea solution is a solution of 32.5% urea in water, where the concentration of the urea solution is strictly restricted, otherwise the NOx emission of a car will exceed the standard and the SCR system of a car may also be damaged.